The present invention relates to microwave antennas, and particularly to planar antennas for circularly polarized waves.
A number of designs have been proposed for high frequency planar antennas, particularly with respect to antennas intended to receive satellite transmissions on the 12 GHz band. One previous proposal is for a microstrip line feed array antenna, which has the advantage that it can be formed by etching of a substrate. However, even when a low loss substrate such as teflon or the like is used, there are considerable dielectric losses and radiation losses from this type of antenna. Accordingly, it is not possible to realize high efficiency, and also when a substrate is used having a low loss characteristic the cost is relatively expensive.
Other proposed antenna designs are a radial line slot array antenna, and a waveguide slot array antenna. These antennas tend to have reduced dielectric and radiation losses, as compared to the microstrip line feed array antenna. However, the structure is relatively complicated, so that production of this antenna design becomes a difficult manufacturing problem. In addition, since each of these designs are formed as a resonant structure, it is very difficult to obtain gain over a wide passband, for example 300 to 500 MHz. Furthermore, these designs are complicated by the cost of coupling between slots, which makes it very difficult to obtain a good efficiency characteristic.
Another proposal is for a suspended line feed aperture array. This design has a structure which overcomes some of the foregoing defects, and can also provided a wide band characteristic, using an inexpensive substrate. Suspended feed line antennas are illustrated in European Patent Application Nos. 108463-A and 123350 and in MSN (Microwave System News), published March 1984, pp. 110-126.
The antenna disclosed in the first of the above applications incorporates copper foils which have to be formed perpendicularly relative to both surfaces of a dielectric sheet which serves as the substrate. Since the structure is formed over both surfaces of the substrate, the interconnection treatment becomes complicated, and the antenna is necessarily relatively large in size.
The antenna disclosed in the other above-cited application requires copper foils to be formed on two separate dielectric sheets. It is difficult to get accurate positioning of these foils, and the construction becomes relatively complicated and expensive. In the antenna disclosed in the MSN publication, one excitation probe is formed in each of a plurality of openings to form an antenna for a linear polarized wave. Such an antenna cannot effectively be used to receive a circular polarized wave, because the gain is poor, and two separate substrates must be used, making the construction relatively complicated and expensive.